In the clinical treatment of patients, the standard means of taking a patient's temperature has been through the use of a mercury-glass thermometer. In recent years, electronic temperature measuring instruments have begun to make inroads into the formerly exclusive domain of the mercury-glass thermometer. Electronic thermometers provide the potential advantages of greater accuracy, if carefully designed; greater ease and efficiency in reading, if digital readout is provided; and faster time response, as compared to a minimum of three minutes for a mercury-glass thermometer. This latter factor can result in greater patient comfort. However, electronic thermometers have not achieved universal acceptance to date, partly because of the inherent advantages in lower initial cost and greater reliability of the mercury-glass thermometers, and also partly because many of the electronic thermometers proposed to date have either failed to live up to the full theoretical advantages potentially offered by electronic devices, or else have done so only at a prohibitively high cost.
Many prior art electronic thermometers use thermistors as their sensing elements. The resistance of the thermistor changes as a function of its temperature in a manner which is predictable and repeatable, but which is unfortunately not a linear function of temperature when measured on the Fahrenheit or Centigrade scale. A wheatstone bridge circuit is often used in conjunction with a servomechanism to measure the resistance changes in the thermistor. A servoamplifier sensing imbalance in the bridge usually drives a motor which drives a variable resistance included in the bridge so as to rebalance the bridge, in response to changes caused by the changing thermistor. Some type of readout driven by the servo gives the temperature readout, through appropriate calibration. Such a readout may be a pointer moving along a dial which has the temperature calibration. Of course, such a dial-type readout is inherently an analog readout and is difficult to read in that it requires careful attention on the part of the operator to read and interpolate the scales correctly, and is additionally subject to parallax errors in viewing the needle. In an attempt to provide for the inherently superior digital readout, some prior art systems have employed an odometer type device which is geared to be run by the servo motor. Of course, such a device is still basically an analog device, and lacks the accuracy and convenience of a true digital readout, because of intermediate positions of numbers on the odometer wheels.
In an attempt to overcome some of these problems in prior art electronic thermometers, fully electronic devices have been proposed which eliminate the mechanical servo, and use all-electronic techniques to measure the variance of the thermistor. Such all-electronic devices can provide the advantages of a truly digital readout, for example, by means of seven segment numerical displays, but they suffer the disadvantages of extremely high cost. The reason for their high cost is that the electronics must not only provide for measurement of the thermistor resistance, but also analog to digital conversion of the thermistor resistance which is inherently an analog quantity, and scale calibration to give the reading in the appropriate terms. Further, if provision is to be made for reading out in either degrees Centigrade or degrees Fahrenheit, which is very desirable and becoming more so all the time, additional scaling circuitry must be provided for this purpose. Needless to say, such additional circuitry increases the cost of the all-electronic thermometers.
Another disadvantage of the all-electronic thermometers is that they do not provide for a nonvolatile memory. It would be desirable in actual clinical use to provide a nonvolatile memory so that the previous measurement would be held in the device until the next measurement is taken. The thermometer operator may wish to reconfirm the previous measurement several minutes or even hours after it was taken, for purposes of documentation. However, in all-electronic thermometers, the digital data, which may be stored in registers, is lost as soon as power is shut off. Of course, memory can be retained in an all-electronic thermometer by leaving the power on, but since for purposes of portability it is desirable that a clinical thermometer be operated on batteries, this would provide unnecessary current drain which would shorten the time between battery chargings.
Another problem to be overcome in electronic thermometers using thermistors as sensing elements is the inherent nonlinearities of the device. Some prior art devices have merely ignored the nonlinearities to result in a simple device, but one with low accuracy. This approach gives up one of the main advantages of the electronic approach. Other prior art devices have used complex electronic function generating devices intended to compensate for the nonlinearities. Such approaches add additional cost and complexity, and may require careful and frequent recalibration to maintain accuracy. Other prior art devices have used nonlinear rebalancing potentiometers in the servo mechanism, characterized so as to make up for the nonlinearities of the thermistor. However, such special potentiometers are expensive, and it is difficult to maintain close manufacturing tolerances on such devices.
Finally, there exists a need in the prior art devices for a truly satisfactory thermistor probe and cooperating sanitary disposable probe cover. Although many such devices generally exist, they have failed to fully meet the requirements of low cost, good thermal conductivity between the probe cover and the thermistor, consistent repeatability in that thermal conductivity from one probe cover to the next, and simple and sanitary one-handed operation in inserting and removing probe covers.
The present invention solves these and other problems existing in the prior art by providing an electromechanical thermometer and probe which is extremely accurate, convenient to use, provides a nonvolatile memory, fully compensates for thermistor nonlinearities, and provides these features at a cost which is much lower than fully electronic devices. Additionally, the present invention provides either Fahrenheit or Centigrade readout, at the flip of a switch, with very little increase in cost or complexity. The present invention also provides an estimation mode of operation, in which the two or so minutes required for a normal reading is reduced to only approximately fifteen seconds, while accuracy is maintained within one-tenth of a degree.